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The Cell Cycle
The Importance of the Cell Cycle
-prokaryotes: simply divide in two
-cell cycle: complex series of stages that eukaryotic cells go through to divide
-unicellular eukaryotes:
-to reproduce
-asexual reproduction
-binary fission
- multicellular:
-growth
-repair
-replacement
-usually develop from a single fertilized egg cell
-human cells have 46 chromosomes
- organism’s cells dividing into many cells  its surface area can keep up with its growing volume
-ex: plants have specialized regions at the tips of their roots and stems
-repeated cell divisions there produce the new cells that develop into the mature tissues of
growing roots, stems, leaves, and other organs
-cell division produces many different types of cells that form the nerves, skin, and other organs
-the timing of cell division is important
-cells in developing tissues pass through the phases of the cell cycle at various rates
-allows cells to make identical copies
-cell division needs accurate replication and equal division of the genetic information encoded in
the cell’s DNA
-each new daughter cell must get an identical set of chromosomes
-an error in DNA replication or cell division can lead to birth defects, cancer, and other serious diseases
-scientists have gained much of what they know about the cell cycle from studies of yeast cells
The Stages of the Cell Cycle
-as a cell completes the cycle, it becomes two new daughter cells
-when a cell divides, its nuclear membrane breaks down
-the individual chromosomes separate and become visible as they are distributed to the daughter cells
-after cytokinesis, each daughter cell enters G1 where they either commit to the full cell cycle or stay in
G0
-different types of cells spend widely different amounts of time in each phase
-cells constantly receive signals from their surroundings which sometimes signal the cell to divide
-when a cell in G0 or G1 gets these signals they pass through the restriction point, R
-this “point of no return” commits the cell to a full round of the cell cycle
-once the cell passes R, it can’t return to G1 or G0 without completing a full cell cycle
-cell size affects the cell cycle alot
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2 main stages:
-interphase: the period between division
-the individual chromosomes are not visible in the nucleus
-3 stages:
1. G1: gap 1 or prereplication
-the cell grows:
-makes more cytoplasm
-increases in size
-as the cell grows larger, the surface area to volume ratio gets smaller
-either prepare for the next change or carry out the cell’s special function
2. S phase: DNA Synthesis
-DNA replicates
-doubles the number of genes in the nucleus
3. G2: gap 2 or premitosis
-organelles within the cell replicate
-and other materials needed for cell division are produced, like RNA, proteins, etc.
-the M phase:
-the cell divides
-2 stages:
-mitosis: nuclear division
-4 major stages: prophase, metaphase, anaphase, telophase
-cytokinesis: cytoplasmic division
-the G0 phase: nondividing cells
-it’s stopping point within G1
-these cells exist, but are stuck here  don’t pass through the rest of the cycle and don’t divide
-skin cells constantly replace themselves, so they are NOT here
-brain and nerve cells don’t replace themselves, so they ARE here
-this is why brain and spinal cord damage can’t be repaired
-liver cells: stays in cell cycle until liver reaches its size  then its cells go into G0
If liver is injured: cells hop back in cycle until the liver once again reaches its size
-most cells in adult multicellular organisms are in G0
-these cells are metabolically active and specialized to perform the tasks necessary to sustain
the life of the organism
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The Race for DNA
-James Watson and Francis Crick
-1953
-were not expected to succeed
-came up with a structural model of DNA that we use today
-used an x-ray crystal picture of DNA that Rosalind Franklin took
-discovered the structure of DNA
-1st attempt: fail
-model: very large, metal
-invited lots of scientists who told them what was wrong
-2nd attempt
-asked Rosalind to work with them
-she got angry and refused to work with them
-Watson saw a picture of DNA that Rosalind took while they were at her place  he knew right away
that DNA is a double helix
Nucleic Acids
-nucleic acids store and transmit genetic information
-DNA, or deoxyribose nucleic acid, is the genetic material that is inherited from one generation to the
next and is reproduced in each cell of an organism
-the instructions in DNA are “copied” to RNA, ribonucleic acid, which directs the synthesis of proteins
-a nucleic acid is a polymer of nucleotides
-a nucleotide is composed of three parts:
1. 5 carbon sugar (pentose)
-either ribose or deoxyribose
2. a nitrogenous base
-there are 4: adenine, thymine, cytosine, guanine
-bases are either single or double rings of carbon, hydrogen, and nitrogen
3. a phosphate group
-has a negative charge
-the sequence of nucleotides in DNA ultimately determines the sequence of amino acids in protein
DNA Structure
-DNA molecules consist of two strands called a double helix
-in 1953, James Watson and Francis Crick proposed this structure
-the backbone, or sides of the DNA molecules are made of sugars and phosphates
-the “rungs” of the DNA molecule are made of 2 nitrogenous bases
-base pairing depends on how many hydrogen bonds each nitrogen base can form with its counterpart
-cytosine only pairs with guanine and thymine only pairs with adenine
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-cytosine only pairs with guanine because 3 hydrogen bonds hold them together, and adenine
only pairs with thymine because two hydrogen bonds hold them together
-purines are paired with pyrimidines
A– – – – – – – T
G – – – – – – –C
-the sugar-phosphate backbones are facing opposite directions
-because the strands are parallel but run in opposite directions, the structure is called antiparallel
-covalent bonds—between sugars and phosphates and between sugars and bases
-hydrogen bonds—between base pairs
-while DNA is a double strand, RNA is a single strand
-DNA forms the chromosomes, units of genetic information, that pass from parent to offspring
-DNA structure:
sugars (deoxyribose)
phosphate group
Nitrogenous bases
adenine = thymine
30%
30%
guanine = cytosine
20%
20%
Erwin Chargaff’s rules:
-each complement base is
equal in amount
-all four together make 100%
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DNA Synthesis
-occurs during the S phase of the cell cycle
-this is a critical step in the cell cycle because it replicates the DNA, so one of each identical
chromosomes can go to the new cells
-the structure of DNA is important in understanding how DNA replicates
-the overall structure of the molecule is a double helix
-the backbone of the molecule consists of alternating sugars and phosphates held together by
covalent bonds
-the rungs of the DNA molecules are composed of nitrogen pairs
-a purine (adenine or guanine) is always bonded to a pyrimidine (thymine or cytosine)
-if u look at the alternating sugar and phosphate bonds on one side of the DNA ladder and compare
them to the opposite side you will see: they are facing in opposite directions—one faces up, the other
faces down
-this structure is referred to as antiparallel and is important in determining the direction in which
the new strands of DNA are synthesized
-the process can be divided into 3 major parts:
1. binding of enzymes to existing DNA
2. unwinding of the double helix
3. synthesis of new matching strand for each existing strand
-these enzymes and proteins are:
-helicase- unwinds double helix
-topoisomerase- prevents tangling of helix
-single strand binding proteins- prevent strands from rejoining
-DNA polymerase
-breaks hydrogen bonds between nitrogenous bases
-brings in new DNA nucleotides
-works 5’ to 3’
-RNA primase:
-puts in 3-5 RNA nucleotides to which the DNA nucleotides can attach
-DNA polymerase-replaces RNA nucleotides
-“proofreads” for errors
*DNA polymerase can’t add or join new DNA nucleotides to something that’s not there  so RNA
primase is used to put in RNA nucleotides that the DNA nucleotides can attach to
-think of it like painting a black table yellow:
-you can’t just paint the table yellow (yellow paint = DNA nucleotides from DNA polymerase)
-you need a white coat called primer (primer = RNA nucleotides from RNA primase)
-the primer isn’t the final color, the yellow replaces the primer (RNA nucleotides get replaced
by DNA nucleotides)
-a replisome consists of all of the enzymes listed above as well as the strand of DNA being copied
-replisomes move in both directions
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-when the DNA molecule “unzips”, it separates into two strands when the hydrogen bonds between
base pairs are broken
-this “unzipping” process occurs in many locations throughout the length of the chromosome
-these specific sites are called replication origins
-multicellular organisms: have many replication origins and replisomes
-bacteria: have one replication origin and replisome
-DNA “unzips” at many places and has many replication origins
-more efficient  replication is faster
-the “unzipping” results in exposed bases on each side of the DNA ladder
-you might expect that new DNA nucleotides would come in to base pair with the exposed bases
-however, the enzyme DNA polymerase can only add new nucleotides to a pre-existing strand of
DNA
-so instead, the enzyme RNA primase first adds a short strand of RNA nucleotides, called a RNA primer,
to being the replication process
-the RNA nucleotides will later be replaced with DNA nucleotides
-once a few nucleotides of FNA primer are in place, the enzyme DNA polymerase begins adding new
DNA nucleotides
-remember: a nucleotide consists of sugar (deoxyribose), a phosphate group, and a nitrogenous
base (A, T, G, or C)
-the nitrogenous base sequence of the existing DNA strand determines the base sequence of the
matching strand
-ex: wherever thymine is on the original strand, adenine is added to the new strand
-the synthesis of the two new sides of the DNA molecule occurs in opposite directions
-always from 5’ to 3’
-one new strand is synthesized continuously into the replication fork  this is the continuous or leading
strand
-the other new strand is synthesized in short segments out of the replication fork  this is the
discontinuous or lagging strand
-these short pieces are called Okasaki fragments and are eventually joined together by an enzyme called
DNA ligase
-the overall process results in two identical copies of the DNA molecule
-this process is called semiconservative replication —each DNA molecule consists of half “old” and half
“new” DNA
-after replication, the DNA strands, called daughter strands, will be more compactly stored in the cell
-to do this, the DNA is wrapped around proteins called histones which are further bundled together to
form nucleosomes
-nucleosomes- a group of usually 8-10 histones
-allows DNA to take up a lot less space
-this coiling results in the highly coiled chromosome structures that are visible during mitosis in the cell
cycle
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DNA replication:
-the DNA molecule unzips
-DNA polymerase adds DNA nucleotides from 5’ to 3’
-DNA polymerase adds new DNA nucleotides
continuously INTO the replication fork on the leading
strand
-DNA polymerase adds new DNA nucleotides OUT of
the replication fork on the lagging strand
-the lagging strand is added in pieces called Okazaki
fragments
-DNA ligase is an enzyme that connects the fragments
of the lagging strand
-the parental strands are the template or mold
-DNA unzips at many replication origins-more efficient: replication is faster
-the replication bubbles expand
-the red = the new strands being added by DNA
polymerase
-the blue = the parental strands
ECT
-Semiconservative replication:
A ---T
G ---C
T ---A
C ---G
parental strand
A
G
T
C
T
C
A
G
the enzyme
helicase unzips the
DNA molecule
A
G
T
C
T
C
A
G
A
G
T
C
T
C
A
G
-DNA polymerase adds new
DNA nucleotides
-two identical strands of DNA:
daughter strands
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DNA Repair
-a mutation occurs when there is any change in the sequence of a cell’s DNA
-mutations may be nonharmful or silent, harmful, or sometimes lethal
-the new DNA strands must be exact complements of the parental strands
-the likelihood of mistakes occurring is reduced because the enzyme DNA polymerase proofreads and
corrects any errors that occur during replication
-the process called excision repair is when the mutations are repaired
-when mistakes are detected, the mistakes are “cut out” and replaced with the correct
nucleotides
-mutagens are chemicals that are environmental factors that cause mutations
-while mistakes can still occur, the proofreading activity by DNA polymerase reduces the number of
mistakes from 1 in 10 thousand base pairs to only 1 in 10 million base pairs
-most mutations are known as mismatches because they consist of base pairs that cannot form
hydrogen bonds (like adenine and cytosine- they can’t pair up no matter what)
-mutations that persist to the next cell division are inherited by the daughter cells
-many scientists believe that the accumulation of many of these mutations over a lifetime may result in
different types of cancer
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Mitosis and Cell Division
The Stages of Cell Division
-it’s important that the process of cell division occur correctly because this process produces 2 new cells
and if it doesn’t, then the cells can be produced incorrectly with mistakes and they won’t survive
-chromosomes are units of genetic information
-they are made of DNA molecules
-chromosomes are found in the nucleus
-when do you call chromosomes what???:
-chromatin: when chromosomes are all tangled together
-during interphase and telophase and cytokinesis
-sister chromatids: when chromosomes are replicated and attached together by a centromere
-during prophase and metaphase
-chromosome: when chromosomes are long and individual
-during anaphase
-there are 46 chromosomes in our cells
-chromosomes are composed of chromatin
-in the early stages of mitosis, the chromatin is condensed and becomes visible, and untangles and coils
back on itself
-chromatin: mostly made of DNA and also protein (histones)
-the DNA is able to fit into the nucleus of the cell:
-the DNA is packaged  the DNA molecules are wrapped around histone proteins which are
grouped together, in groups of 8-10 histones, called nucleosomes
-the centromere joins together the sister chromatids
-it’s found at the center of the chromosomes
-separation of the chromatids is called chromosome segration
-if segregation occurs correctly, each new nucleus receives one copy of each chromosome
-a mistake at this stage would be one nucleus with two chromosomes and the other nucleus with
none  daughter cells with this mistake are celled aneuploid cells
-Interphase
(G1—S—G2)
-chromosomes appear in the form of chromatin— appears as a dark granular mass, so you can’t really
see the chromosomes
-chromatin: mostly DNA and proteins (histones)
-chromosomes have replicated during the S phase
-the 3 events:
-G1-prereplication
-S-DNA synthesis
-G2- premitosis
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-the 4 stages of mitosis:
-the result of mitosis is the production of two nuclei each with a duplicate set of chromosomes
-scientists still don’t completely understand how chromosomes move during mitosis
1. prophase
-the longest phase of mitosis
-begins when the nuclear membrane breaks down into small vesicles
-early prophase:
-the chromatids become visible because the chromatin has condensed and thickened
-the chromosomes appear as two identical sister chromatids
-sister chromatids are attached at the centromere
-contents of the nucleolus and nuclear membrane disperse and they appear to disappear
-centrioles, which are normally found outside the nucleus, separate and move to opposite
sides of the nucleus
-the centrioles contain tubulin, a microtubule protein
-late prophase:
-the spindle, a network of proteins that helps to move the chromosomes apart, is produced
from the centrioles
-spindle fibers begin to attach to the sister chromatids at their kinetochores
-near the end of prophase, the coiling of the chromatids becomes tighter
-sister chromatids appear short and thick  coiled back on themselves so it’s easier to move
around
*even though plants don’t have centrioles, they still produce spindle fibers which help to pull
the chromosomes apart
2. metaphase
-the shortest phase of mitosis
-sister chromatids align at the equator or metaphase plate
-spindle fibers attach to the kinetochores of each chromatid
-the spindle, which are arranged in the starlike pattern around the poles of the spindle, are often
called asters which is the greek word for star
3. anaphase
-sister chromatids separate as spindle fibers shorten
-begins when centromeres that join the sister chromatids spit
-this causes the chromatids to split and form separate chromosomes
-the chromosomes continue to move until they have separated into two groups
-each group is now found near the poles of each of the spindles
-ends when the chromosomes have stopped moving
4. telophase
- chromosomes uncoil to form a tangle of chromatin
-this occurs in two regions—where the nuclei of the daughter cells will form
-the nuclear envelope reforms around the chromatin
-the spindle breaks apart and the nuclear envelope and nucleolus once again become visible
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-this marks the end of mitosis
Cytokinesis
-often occurs during telophase
- the cytoplasm of the cell divides
-this results in the production of two complete and individual daughter cells
-cytokinesis is different in plant and animal cells:
-plants:
-cell plate forms from the center of the cell outward
-plants don’t have centrioles
-animals:
-cleavage furrar: how and where the cytoplasm divides
Differences in Mitosis
-although the major events of cell division are similar in all eukaryotic cells, there are subtle differences
-cytokinesis begins during anaphase in most animal cells
-plants don’t have centrioles
-the biggest difference between cell division in animal and plant cells is due to the fact that plants have
cell walls
-at cytokinesis in plants, vesicles containing cellulose begin to gather between the two nuclei
-these vesicles then begin to fuse, forming the plasma membrane of the two new daughter cells
-the contents of the vesicles complete the cell wall between the two new cells
-in some fungi like yeast, the nuclear envelope forms a bud instead of breaking down, and the spindle
poles are embedded in the nuclear membrane
-Key Terms
-cell division: the process by which a cell divides into two daughter cells
-mitosis: process by which the nucleus of a cell is divided into two nucei, each with the same number
and kinds of chromosomes as the parent cell
-cytokinesis: process by which the cytoplasm divides into two daughter cells
-chromosome: threadlike structure in a cell that contains the genetic information that is passed on from
one generation of cells to the next
-chromatin: a tangled mess of chromosomes that is composed of DNA and proteins
-chromatids: two identical chromosomes attached at their centromere
-centromere: structure that holds together each pair of chromatids
-cell cycle: period from the beginning of one mitosis to the beginning of the next
-interphase: period of the cell cycle between cell divisions
-kinetochore: the protein on the centromere for spindle fibers to attach to
-prophase: first phase of mitosis in which the chromatids become visible
-centriole: structure involved in mitosis that contains a microtubule protein called tubulin
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-spindle: meshlike structure of centrioles that appears to guide the movement of chromosomes during
mitosis
-metaphase: second phase of mitosis in which the chromatids line up across the equator
-anaphase: third phase of mitosis in which sister chromatids separate
-telophase: final phase of mitosis where the chromosomes uncoil to form chromatin, and the nuclear
envelope and nucleolus reform in each daughter cell
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